Best of the series!
The addition of deployable troops really makes Retribution shine - the RPG elements are still there but now you've got cannon fodder to throw at your enemy. I especially love the high squad counts of the Chaos and Guard factions - the carnage is lovely.
The capper for this game, imho, is the scope! There are campaigns for EACH of the 6 playable races. The first several missions are mostly identical but it doesn't matter! The unique play-styles of each race make even the same level a novel experience. Over the past several nights I have systematically started a campaign for each race, getting them all to the third mission, so I've gotten very fast at those first few missions but I never got bored. I haven't gotten any further than that and I'm already looking forward to replaying the game.